May 23, 2023
Experimental Populations Under the Endangered Species Act
Section 10(j) of the Endangered Species Act (ESA; 16
has been “irreversibly altered or destroyed” so as to render
U.S.C. §§ 1531 et seq.) allows for the establishment of
it unsuitable for the species.
experimental populations of endangered or threatened
species listed under the act and delineates how they are to
Wholly Separate Geographically
be regulated. Congress added Section 10(j) to the ESA as
The Services’ regulations provide that the geographic
part of the Endangered Species Act Amendments of
separation between the released population and existing
1982.The amendments gave the U.S. Fish and Wildlife
populations must be “reasonably predictable.” For example,
Service (FWS) and National Marine Fisheries Service
fixed migration patterns or natural or manmade barriers
(NMFS) (together, the Services) more flexibility to
between the populations may create sufficiently predictable
determine how to protect and manage experimental
separation. When making this determination, the Services
populations, and they altered certain agency consultation
focus on the population as a whole rather than individuals
requirements for nonessential experimental populations.
within the population. Individuals from an experimental
population that venture into an area of overlap with the non-
With certain exceptions, experimental populations are
experimental population are not recognized as part of the
treated as threatened species under the ESA regardless of
experimental population while they are intermixed with a
the designation of the listed species generally. This
non-experimental population.
treatment allows the Services to tailor the protections and
exceptions that apply to the population, providing for
Contribution to the Conservation of the Species
flexible management while contributing to the conservation
The ESA requires that the establishment of an experimental
of the species.
population must contribute to the conservation of the listed
species. The Services must consider whether removing
The Services have established more than 60 experimental
individuals from existing populations to establish the
populations for many kinds of species. Examples include
experimental one will adversely affect those populations.
the gray wolf, grizzly bear, black-footed ferret, California
They must also consider whether the experimental
condor, Chinook salmon, Anthony’s riversnail, and
population is likely to survive for the foreseeable future and
American burying beetle. Two experimental populations of
meet population goals. The agencies must further examine
gray wolf that were released in Idaho, Montana, and
the anticipated effects that establishing an experimental
Wyoming in the mid-1990s eventually allowed the resultant
population will have on the species’ recovery. Finally, the
distinct population segment to be delisted through
Services must determine whether and to what extent
administrative and legislative action.
existing or anticipated federal or state actions or private
activities may be affected by establishing an experimental
This In Focus describes the criteria and process for
population. Each factor must be determined based on the
establishing and regulating 10(j) experimental populations.
best commercial and scientific data available.
Experimental Population Criteria
Designation as Essential or Nonessential
Section 10(j) authorizes the Secretary of the Interior or of
When the Services designate an experimental population,
Commerce, as delegated to FWS or NMFS, respectively, to
Section 10(j) of the ESA also requires that they determine
release endangered or threatened species into the wild
whether the experimental population is “essential to the
outside the current range of the species, provided the
continued existence” of the species. The Services designate
Services determine that the release will contribute to the
such experimental populations as essential experimental
conservation of the species. Experimental populations must
populations and all others as nonessential experimental
be wholly separate geographically from any non-
populations. These determinations are made based on the
experimental populations of the species. The Services have
“best available information.” Pursuant to their regulations,
each enacted regulations to implement Section 10(j).
the Services consider an experimental population to be
essential if losing the population would likely “appreciably
Outside the Species’ Current Range
reduce the likelihood” of the species surviving in the wild.
Section 10(j) requires that the Services release experimental
To date, no experimental population has been designated as
populations outside the species’ current range. FWS
essential.
regulations further require the experimental population to
be released into “suitable natural habitat.” In addition, FWS
Implications of Designation
generally requires the release to be within the species’
In general, even if a species is listed as endangered,
historical range except in the “extreme case” where FWS
experimental populations of that species are treated as
concludes that the species’ habitat within its historical range
threatened species under the statute. The prohibitions in
Section 9 of the ESA, such as on importing or exporting
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Experimental Populations Under the Endangered Species Act
species, automatically apply to endangered species, but they
The Services’ regulations require rules establishing
only apply to threatened species (and, by extension,
experimental populations to include certain information in
experimental populations) if provided by regulation. When
addition to demonstrating that the released population meet
the Services extend Section 9 prohibitions to experimental
the statutory criteria. The rule must provide a means to
populations, those regulations are referred to as 10(j) rules.
identify the experimental population, such as specifying the
proposed location or anticipated migration. It must identify
For threatened species, the Services must designate critical
any special management concerns, such as management
habitat to the maximum extent prudent and determinable.
restrictions or protective measures to isolate or contain the
Critical habitat generally consists of habitat that is essential
population from the rest of the species. It must also create a
to the conservation of the species. Section 10(j) allows the
process for periodic evaluation of how effective the
Services to designate critical habitat for essential
experimental population is at conserving the species.
experimental populations but precludes such designations
for nonessential experimental populations.
For an essential experimental population, the agency may
also designate critical habitat. The Service may also choose
ESA Section 7 (16 U.S.C. § 1536) generally requires
to implement a 10(j) rule or designate critical habitat as part
federal agencies to consult with the Services when their
of the rule establishing the experimental population or
actions may affect endangered or threatened species. Under
through a separate rulemaking process.
Section 10(j), however, nonessential experimental
populations are generally subject instead to the Section 7
Judicial Review of Designation and 10(j) Rules
requirements that apply to species that are proposed to be
Both the ESA and the APA provide a basis for judicial
listed. For proposed species, Section 7 requires federal
review of the Services’ final designation and 10(j) rules for
agencies to confer with the Services about actions that may
experimental populations, depending on the plaintiffs’
affect the species, which is a more informal process than
particular allegations. The ESA allows citizens to sue in
consultation that allows the agency to proceed with its
federal district court to enforce provisions of the ESA or its
action in the meantime and results in only advisory
regulations. That includes suits claiming that the Services
recommendations about minimizing adverse effects to the
have violated Section 10(j) or challenging regulations
species. For proposed actions within the National Park
issued under Section 10(j).
System or the National Wildlife Refuge System, Section 7
applies to nonessential experimental populations as it would
In reviewing those claims, courts apply the “arbitrary and
to threatened species.
capricious” standard contained in the APA. The ESA,
however, does not provide for suit against the Services in
Process for Designating Experimental
certain instances. Some challenges to the Services’
Populations and Crafting 10(j) Rules
administration of the ESA through the rulemaking process
The Services establish experimental populations and
must proceed under the APA, not the ESA.
protections for those populations through the federal
rulemaking process. Federal agency rulemaking procedures
Considerations for Congress
are largely governed by the Administrative Procedure Act
Although Section 10(j) affords the Services more flexibility
(APA). The Services’ designations and 10(j) rules are thus
to manage experimental populations than other listed
governed primarily by requirements established in the ESA,
populations, some stakeholders still raise concerns that
implementing regulations, and the APA.
designating an experimental population may affect
development in particular areas. In addition, experimental
Issuing Designation and 10(j) Rules
populations of large predators such as gray wolves and
To designate an experimental population, the Service first
grizzly bears can raise conflicting views about possible
publishes a proposed rule providing notice of its intent to
interactions with livestock and big game herds.
designate an experimental population, its proposed
designation of essential or nonessential, and any proposed
For example, FWS’s proposal to release an experimental
10(j) rule for the population.
population of gray wolves in Colorado in 2023 has raised
this issue. At the same time, other stakeholders criticize
The Services’ regulations require the agencies to consult
10(j) rules that permit killing or harassing experimental
with relevant state fish and wildlife agencies and local
populations of large predators, claiming those rules fail to
governmental entities, as well as affected federal agencies
fulfill the goals of the ESA, limit the species’ ability to fully
and private landowners, when developing and
recover, and prevent the species from reestablishing natural
implementing experimental population regulations. This
relationships with large ungulate populations.
consultation process may involve holding public meetings.
Likewise, the APA requires the Services to provide an
Congress may consider whether to leave such decisions to
opportunity for the public to comment on the proposal.
the discretion of the agency or to provide more direction as
After considering any public comments, the Service may
to how experimental populations should be managed, either
choose to complete the rulemaking process by publishing a
in general or for specific species. Congress may also
final rule. The final rule must provide a reasoned
consider legislation directly targeting stakeholder concerns
justification supporting the agency’s action, including
outside the 10(j) context, such as through depredation grant
responses to significant comments.
programs for livestock owners.
Erin H. Ward, Legislative Attorney
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Experimental Populations Under the Endangered Species Act

IF12407
Benjamin M. Barczewski, Legislative Attorney


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https://crsreports.congress.gov | IF12407 · VERSION 1 · NEW

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